Groundwater, enhanced through managed aquifer recharge (MAR), plays a central role in mitigating current and future water stress. Here we evaluate anthropogenic and natural water isotopes as tracers of groundwater flow dynamics within alluvial MAR systems. High-resolution sampling (daily/weekly) of stable isotopes (δ¹⁸O and δ²H) and tritium (³H), influenced by nuclear power plant effluents, is used to trace and quantify the movement of infiltrated river water through an alluvial aquifer along the Rhine River in Switzerland. Time-series deconvolution is applied to quantify the tracer-based travel time distribution and to predict travel times throughout the entire MAR scheme. The results demonstrate the suitability of ³H as a quasi-conservative travel time tracer in systems where the infiltrating river water is marked by nuclear power plant discharges—a situation prevalent along the banks of many large river basins globally. Deuterium excess proved equally effective as a bulk travel time tracer, reflecting distinct seasonal meltwater signals expected in major European rivers. These findings quantify MAR recovery rates and wellhead protection zones, supporting sustainable groundwater management under natural and anthropogenic pressures.

Alluvial systems are dynamic, structurally complex environments. Groundwater that is extracted from these systems is widely used for domestic consumption. Numerical models play an important role in the management of this extraction. To provide support for decision-making, models must assimilate data that inform the spatial and temporal dynamics of the interactions between alluvial surface water (SW) and groundwater (GW). In theory, assimilation of noble gas tracer data into numerical models that solve the advection-dispersion equation as it pertains to gas tracer concentrations can reduce the uncertainties of management-salient predictions made by these models. To date, however, this has not been undertaken; nor has its decision-support efficacy been tested. A modeling framework is introduced whereby data space inversion, a highly efficient method for evaluation of post-data-assimilation uncertainties of complex simulators, is used to evaluate benefits accrued through assimilation of noble gas tracer concentrations (i.e., ²²²Rn; ³⁷Ar; ⁴He) along with more classical observations. The ability of noble gas concentrations to reduce the uncertainties of predictions made using an integrated surface-subsurface hydrological model is rigorously evaluated in simulation experiments that span a range of plausible hydraulic and hydrological conditions. Outcomes of these experiments demonstrate that measurements of ²²²Rn and ⁴He concentrations are able to constrain temporally-variable predictions, such as the magnitudes of SW-GW exchange fluxes, as well as solute breakthrough curves that reflect the residence times of potential pollutants following an aquifer contamination event. ³⁷Ar concentrations can also provide important information on SW-GW interactions, especially the residence times of bank filtrate.

Dissolved noble gases have been recognized for decades as ideal artificial hydro(geo)logical tracers, as they are chemically inert, invisible, and non-toxic. However, their widespread adoption has historically been limited by the difficulty of tracer injection, sampling, and analysis procedures. Developments in portable, high-resolution dissolved gas measurement technology over the last two decades have rekindled interest in the use of gas tracer methods for routine hydrogeological investigations, such as well-to-well tracer tests, intra-well tests, or studies of river infiltration towards alluvial aquifers. The application of gases in aqueous environments still poses unique challenges compared to other tracer methods, as potential exsolution and degassing need to be accounted for, and, if possible, avoided during tracer injection. Here, we present a simple and efficient methodology that addresses these challenges and allows the efficient, on-site preparation and injection of highly concentrated tracer solutions with controlled dissolved gas concentrations. We applied the method in a large drinking water wellfield and performed well-to-well tracer tests in an unconfined aquifer using helium-4 (⁴He), neon-20 (²⁰Ne) and krypton-84 (⁸⁴Kr). Known tracer quantities were injected together with fluorescent dyes into an observation well upgradient of a pumping well. Gas tracer breakthrough was monitored in the pumping well with a portable mass spectrometer. Breakthrough curves of ⁴He and ⁸⁴Kr compared favorably with fluorescent dye tracers, and enabled reliable estimates of groundwater flow velocities, travel times, and tracer recovery. These findings illustrate how noble gases can substitute or complement other artificial tracer methods, even in large-scale settings. The methodology can be extended to other gases (e.g., neon-22, xenon isotopes, light hydrocarbons), significantly expanding the range of artificial tracers available for routine hydrogeological investigations.

This dataset provides high-resolution 3D geological and integrated hydrological models of Mt. Fuji watershed in Japan. The watershed’s complex volcanic and tectonic setting, large spatial extent, and limited subsurface data present significant challenges for integrated hydrological modeling. Diverse geological datasets – borehole logs, geological maps, and hydrofacies surfaces – were collected, processed, and used to construct and validate a 3D geological model suitable for integrated hydrological simulations. Building on this, a 3D numerical model for integrated hydrological simulations was constructed. The repository includes 3D hydrofacies surfaces in raster format, numerical mesh files, and input configurations necessary to run simulations with the integrated surface-subsurface hydrological simulator HydroGeoSphere. The preparation of heterogeneous geological data, construction of hydrofacies surfaces, generation of the numerical mesh, and setup of the integrated hydrological model are described in a streamlined, reproducible workflow suited for volcanic contexts and transferable to other geologically complex or data-limited regions. These resources are intended to reduce trial-and-error iterations and support further research in groundwater assessment, model calibration, climate impact studies, and hazard mitigation.

Mountainous regions such as Switzerland, which are characterized by highly complex geological and hydrogeological conditions, hold significant geothermal potential linked to tectonic settings and deep hydrothermal flow systems. However, geothermal exploration in such regions is challenged by geological and hydrogeological complexity, data scarcity, high uncertainty, administrative hurdles, as well as environmental and economic risks associated with drilling. Robust quantitative tools that maximize the value of the limited information available on the subsurface are needed to assess geothermal potential, productivity, and sustainability. Here, a step-by-step framework for the assessment of regional hydrothermal flow systems is presented. The framework consists of 3D geological modeling, 3D thermohydraulic modelling (3D-TH-model), and a systematic sensitivity analysis for the identification of the optimal structural model complexity. The framework is demonstrated on the hydrothermal systems of the Upper Aarmassif in the Swiss Alps. Results showed that thermal upwelling in the system is driven by strong topographic gradients and upward flux along more permeable tectonic faults and thrusts. The upwelling results in significantly elevated groundwater temperatures near the surface in the Rhône Valley, with temperatures exceeding 70 °C at depths of 1000 m and reaching ~100 °C at depths of 2000 m. These model outputs were verified by the few available vertical borehole temperature profiles, horizontal tunnel temperature profiles, groundwater recharge rates and hydraulic head distribution, ¹⁴C-based groundwater residence times, and geothermometrically identified hydrothermal-reservoir-temperature estimates. The framework provides the basis for risk-reduced geothermal exploration, thereby supporting sustainable geothermal development—a significant step towards a decarbonized energy future.

Assessing the transport behaviour of microbes in surface water–groundwater systems is important to prevent contamination of drinking-water resources by pathogens. While wellhead protection area (WHPA) delineation is predominantly based on dye injection tests and advective transport modelling, size exclusion of colloid-sized microbes from the smaller and usually less conductive pore spaces causes a faster breakthrough and thus faster apparent transport of microbes compared to that of solutes. To provide a tool for better assessment of the differences between solute and microbial transport in surface water–groundwater systems, here, we present the implementation of a dual-permeability, two-site kinetic deposition formulation for microbial transport in the integrated surface–subsurface hydrological model HydroGeoSphere (HGS). The implementation considers attachment, detachment, and inactivation of microbes in both permeability regions and allows for multispecies transport. The dual-permeability, two-site kinetic deposition implementation in HGS was verified against an analytical solution for dual-permeability colloid transport. The suitability of the model for microbial transport in integrated surface–subsurface hydrological settings at the wellfield or small headwater catchment scale is demonstrated by two illustrative examples. The first example is a benchmark for integrated rainfall–runoff and streamflow generation modelling to which we added microbial transport from a conceptual manure application, demonstrating the novelty of explicit and coupled microbial and solute transport simulations in an integrated surface–subsurface hydrological scenario. The second example is a multi-tracer flow and transport study of an idealized alluvial riverbank filtration site, in which we simulate in parallel the transport of reactive microbes, conservative ⁴He, and reactive ²²²Rn, demonstrating the assessment of mixing ratios, tracer breakthrough curves, and travel times in an integrated manner via multiple approaches. The developed simulation tool represents the first integrated surface–subsurface hydrological simulator for reactive solute and microbial transport and marks an important advancement to unlock and quantify governing microbial transport processes in coupled surface water–groundwater settings. It enables meaningful WHPA delineation and risk assessments of riverbank filtration sites with respect to microbial contamination even under situations of extreme hydrological and microbial stress, such as flood events.

Der Klimawandel führte bereits nachweislich zu einem Anstieg der Wassertemperaturen in Schweizer Fliessgewässern. Dieser Trend dürfte sich fortsetzen – wie stark, hängt von den Klimaschutzmassnahmen ab. So werden ohne entsprechende Massnahmen mittlere Fliessgewässertemperaturen bis Ende des 21. Jahrhunderts um 3,11 ± 0,7 °C ansteigen, mit Massnahmen werden es 0,9 ± 0,3 °C sein. Die stärkste Erwärmung ist bei Fliessgewässern im alpinen Raum zu erwarten, gefolgt von Fliessgewässern unterstromig von Seen. Dies ergaben systematische Untersuchungen im Rahmen des Forschungsprojekts «Swiss Future River Temperatures – SwissFuRiTe».

The influence of seasonal variations in temperature and precipitation on subsurface biogeochemical processes remains poorly understood. In the Lavey-les-Bains thermal system in the Swiss Alps, annual variations in electrical conductivity are observed to depths of 500 m, suggesting a potential link to surface environmental changes. Here we show, through year-round analyses of stable water isotopes, noble gases, and conductivity, that seasonally varying contributions of shallow groundwater from the Rhône alluvial aquifer mix with deep groundwater. Despite vertically similar fluid geochemical compositions suggesting high hydrological connectivity, microbial communities exhibit significant depth-dependent variation with minimal seasonal change. This decoupling of dynamic water source partitioning and stable microbial community structure has not been previously observed and fills a critical gap in our understanding of geothermal systems and microbial life in the deep subsurface. At 200 m, the communities are dominated by sulfur-disproportionating Bacteria (Dissulfurispira) and Micrarchaeota, while at 500 m the major groups include sulfate- and iron-reducers and/or hydrogen-oxidizers (Thermales, Thermodesulfobacteriota, and Bathyarchaeota). Our study highlights the resilience of terrestrial subsurface microbial communities to temporal variations in water sources and fluid composition. We propose that intrinsic environmental properties - such as temperature - are more critical drivers of microbial community structure in hydrologically connected deep aquifers than seasonal hydrological changes.

Environmental DNA (eDNA) has revolutionized ecological research, particularly for biodiversity assessment in various environments, most notably aquatic media. Environmental DNA analysis allows for non-invasive and rapid species detection across multiple taxonomic groups within a single sample, making it especially useful for identifying rare or invasive species. Due to dynamic hydrological processes, eDNA samples from running waters may represent biodiversity from broad contributing areas, which is convenient from a biomonitoring perspective but also challenging, as hydrological knowledge is required for meaningful biological interpretation. Hydrologists could also benefit from eDNA to address unsolved questions, particularly concerning water movement through catchments. While naturally occurring abiotic tracers have advanced our understanding of water age distribution in catchments, for example, current geochemical tracers cannot fully elucidate the timing and flow paths of water through landscapes. Conversely, biological tracers, owing to their immense diversity and interactions with the environment, could offer more detailed information on the sources and flow paths of water to the stream. The informational capacity of eDNA as a tracer, however, is determined by the ability to interpret the complex biological heterogeneity at a study site, which arguably requires both biological and hydrological expertise. As eDNA data has become increasingly available as part of biomonitoring campaigns, we argue that accompanying eDNA surveys with hydrological observations could enhance our understanding of both biological and hydrological processes; we identify opportunities, challenges, and needs for further interdisciplinary collaboration; and we highlight eDNA's potential as a bridge between hydrology and biology, which could foster both domains.

The mountain cryosphere and groundwater play pivotal roles in shaping the hydrological cycle, yet their connectivity remains incompletely understood. Current knowledge on meltwater recharge and consequent groundwater discharge processes is better developed for snow–groundwater connectivity than for glacier–groundwater connectivity. Estimates of meltwater recharge vary considerably, which is probably a function of not only inherent catchment characteristics but also of the different spatio-temporal scales involved and the uncertainties in the methods used. This hinders a comprehensive understanding of the mountain water cycle. As glaciers retreat, permafrost thaws and snowpack diminishes, the relative importance of mountain groundwater is expected to increase. However, shifting and declining recharge from the cryosphere may decrease absolute groundwater amounts and fluxes with as-yet unknown effects on catchment-scale hydrological processes. We therefore stress the need to better quantify mountain cryosphere–groundwater connectivity to predict climate change impacts on mountain water supply and to support sustainable water resource management of downstream socio-ecological systems.

Known locally as the water mountain, for millennia Japan’s iconic Mt Fuji has provided safe drinking water to millions of people via a vast network of groundwater and freshwater springs. Groundwater, which is recharged at high elevations, flows down Fuji’s flanks within three basaltic aquifers, ultimately forming countless pristine freshwater springs among Fuji’s foothills. Here we challenge the current conceptual model of Fuji being a simple system of laminar groundwater flow with little to no vertical exchange between its three aquifers. This model contrasts strongly with Fuji’s extreme tectonic instability due to its unique location on top of the only known continental trench–trench–trench triple junction, its complex geology and its unusual microbial spring water communities. On the basis of a unique combination of microbial environmental DNA, vanadium and helium tracers, we provide evidence for prevailing deep circulation and a previously unknown deep groundwater contribution to Fuji’s freshwater springs. The most substantial deep groundwater upwelling has been found along Japan’s most tectonically active region, the Fujikawa-kako Fault Zone. Our findings broaden the hydrogeological understanding of Fuji and demonstrate the vast potential of combining environmental DNA, on-site noble gas and trace element analyses for groundwater science.

Zur Beurteilung des hydraulischen und thermischen Einflusses von Verkehrsinfrastrukturen und Tiefbauten auf Untergrundressourcen bedarf es einer geologischen, hydrogeologischen und geotechnischen Begleitung. Exemplarisch wird für mehrere in der Schweiz geplante Infrastrukturprojekte (Rheintunnel Basel, S-Bahntunnel «Herzstück», Tiefgaragen sowie «Cargo sous terrain» im Mittelland) illustriert, wie komplexe geologische und hydrogeologische Randbedingungen berücksichtigt und eine nachhaltige Nutzung von Untergrundressourcen realisiert werden können.

As climate change adaptation strategies, both Managed Aquifer (MAR) and Surface Water Recharge (MSWR) are not only highly suitable tools to mitigate negative effects on water resources but also bear large potential for concomitant exploitation of thermal energy. They should thus form an integral part of any sustainable water resources management strategy. However, while at global scale general water resource adaptation and mitigation measures are discussed widely, measures that build on thermal exploitation of MAR and MSWR, and which are readily adaptable to various different local and regional scale conditions, have yet to be developed.
Here, based on systematic numerical analyses of the sensitivity of groundwater and surface water recharge as well as water temperatures to climate change, we present adaptable implementation strategies of MAR and MSWR with concomitant exploitation of their thermal energy potential. Strategies and feasibility benchmarks for the exploitation of hydrologic and energetic potentials of MAR and MSWR were developed based on three hydrologically and hydrogeologically contrasting urban study sites near the city of Basel, Switzerland. Our studies show projected trends in the number of days when surface water temperatures exceed 25 °C examined for various streamflow and climate scenarios.
We illustrate that local hydrogeologic settings and hydrological boundary conditions as well as legal aspects affect to which degree MAR and MSWR are suitable solutions as climate change adaptation measures. Optimal situations for exploiting the potential of seasonal heat storage in MAR and MSWR exist where subsurface travel times between the injection and the withdrawal or exfiltration point are between 4 and 8 months and legal limits allow a sufficiently large temperature spread. In such settings, the exploitable water flux and temperature spread of MAR and MSWR reaches a heat potential of 14 to 20 MW (i.e., corresponding to 3 to 7 wind power plants), and energetic exploitation becomes a suitable tool either for local low-temperature heat applications such as heating and hot water or for ecological use as a heat and water buffer in rivers affected by seasonal droughts. As a positive side effect, climate-induced warming of groundwater resources and temperature increases in drinking water withdrawals would be mitigated simultaneously.

Grundwasser ist eine wichtige natürliche Ressource, die für die Trinkwasserversorgung, menschliche Aktivitäten, die Landwirtscha und das Funktionieren der Ökosysteme eine entscheidende Rolle spielt. Allerdings stehen die Grundwasserressourcen in der Schweiz aufgrund des Klimawandels vor grossen Herausforderungen. Der vorliegende Artikel gibt Einblick in die vorhergesagten Veränderungen.

Als Strategien zur Anpassung an den Klimawandel eignen sich sowohl die thermische Bewirtschaftung von Grundwasserleitern als auch von Oberflächengewässern. Einerseits lassen sich damit negative Auswirkungen des Klimawandels auf die Wasserressourcen abschwächen. Andererseits besteht ein grosses Potenzial für die gleichzeitige Nutzung von Wärmeenergie. Deshalb sollten Klimawandel-Anpassungsstrategien ein integraler Bestandteil der nachhaltigen Bewirtschaftung von Wasserressourcen sein.

Quantitative Abschätzungen der Auswirkungen des Klimawandels auf Grundwasserressourcen erlauben es, zwischen hydraulischen und thermischen Auswirkungen auf natürliche und künstliche Grundwasserneubildung zu differenzieren. In Zukunft werden vermehrt Auswirkungen von saisonalen Verschiebungen bei der natürlichen Grundwasseranreicherung sowie Anpassungsstrategien der Wasserversorgung zu beobachten sein. Dabei ist zu berücksichtigen, dass einzelne Trinkwasserbrunnen von unterschiedlichen Komponenten der Grundwasserneubildung betroffen sind. Darüber hinaus dürften eine zunehmende künstliche Grundwasseranreicherung in den Sommermonaten und die Interaktion mit Oberflächengewässern während Hochwasserereignissen, die in den Wintermonaten häufiger auftreten werden, die Grundwasseranreicherung und -temperaturen stark beeinflussen.

Les évaluations quantitatives des effets du changement climatique sur les ressources en eaux souterraines permettent de différencier les effets hydrauliques et thermiques sur la recharge naturelle et artificielle des nappes phréatiques. A l’avenir, les effets des décalages saisonniers sur la recharge naturelle des eaux souterraines et les stratégies d’adaptation pour l’approvisionnement en eau seront de plus en plus observés. Il faut tenir compte du fait que les puits d’eau potable individuels sont affectés par différentes composantes de la recharge des eaux souterraines. En outre, l’augmentation de la recharge artificielle des nappes phréatiques durant les mois d’été et l’interaction avec les eaux en surface lors des crues, qui se produiront plus fréquemment pendant les mois d’hiver, devraient fortement affecter la recharge et les températures des eaux souterraines.

Environmental gas tracers allow inferring groundwater travel times and mixing ratios. Their concentrations are commonly interpreted with simplified and indirect approaches that are conceptually at odds with the high degree of complexity found in natural systems. However, the information content of the tracers can potentially be fully explored through the explicit simulation of an advection-dispersion transport equation, for example using integrated surface-subsurface hydrological models (ISSHMs). These integrated models can be used to explicitly simulate environmental tracers in complex environments. ISSHMs are usually variably saturated flow models. However, these models do not explicitly simulate gas partitioning with the aqueous phase, restricting explicit simulation of gas tracers to fully saturated conditions or to tracers with very low solubilities. We propose a mathematical formulation for the production of environmental gas tracers that are emanated in the subsurface. The production is scaled according to gas/water partitioning and water saturation, which is already computed by the model. Therefore, ISSHMs can now be used to their full potential to explicitly simulate tracer concentrations under variably saturated and dynamic conditions. The new formulation has been successfully verified against reference simulations provided with a multi-phase flow and transport model. In addition, explicit simulation of ²²²Rn and ³⁷Ar groundwater concentrations in a synthetic alluvial river-groundwater system was demonstrated, for the first time, with an ISSHM.

Für die Erstellung multipler Grundwassermodelle und deren Unsicherheitsanalyse existiert zwar eine Vielzahl an Methoden, aber in der Praxis bleibt dieses Potenzial oftmals ungenützt. Da Standard-Grundwassermessdaten nicht genügend Informationen für das Identifizieren hydraulischer Parameter beinhalten und auch das Hinzuziehen von unkonventionellen Messdaten Parameterunsicherheiten nicht komplett eliminiert, ist eine Unsicherheitsanalyse von Grundwassermodellen unabdingbar. Dieser Artikel zeigt Vorteile und Wege auf, wie dies in der Praxis umgesetzt werden kann.

Alluvial sand and gravel (ASG) aquifers are highly heterogeneous and exhibit strong, spatially variable anisotropy, often interspersed by buried paleo-channels of increased hydraulic conductivity. Groundwater flow and solute transport is often characterized by preferential flow caused by anisotropic properties in ASG aquifers. Connected ASG subsurface structures such as buried paleo-channels, however, are difficult to reproduce with commonly used techniques, and anisotropy is rarely considered in applied groundwater models. To ease the notoriously difficult problem of how to consider anisotropy, we propose a novel modeling framework based on calibration of an integrated surface-subsurface hydrological model via spatially varying, preferred anisotropy pilot point inversion. The inversion leverages hydraulic and tracer-based observations representing multiple spatial and temporal scales. We demonstrate the applicability of the framework on a real-world ASG site used for drinking water production, and we quantify the information content of observations to identify connected paleo-channels and provide guidance for optimal field-data acquisition.

Understanding the mixing between surface water and groundwater as well as groundwater travel times in vulnerable aquifers is crucial to sustaining a safe water supply. Age dating tracers used to infer apparent travel times typically refer to the entire groundwater sample. A groundwater sample, however, consists of a mixture of waters with a distribution of travel times. Age dating tracers only reflect the proportion of the water that is under the dating range of the used tracer, thus their interpretation is typically biased. Additionally, end-member mixing models are subject to various sources of uncertainties, which are typically neglected. In this study, we introduce a new framework that untangles groundwater mixing ratios and travel times using a novel combination of in-situ noble gas analyses. We applied this approach during a groundwater pumping test carried out in a pre-alpine Swiss valley. First, we calculated transient mixing ratios between recently infiltrated river water and regional groundwater present in a wellfield, using helium-4 concentrations combined with a Bayesian end-member mixing model. Having identified the groundwater fraction of recently infiltrated river water (F_rw) consequently allowed us to infer the travel times from the river to the wellfield, estimated based on radon-222 activities of F_rw. Furthermore, we compared tracer-based estimates of F_rw with results from a calibrated numerical model. We demonstrate (i) that partitioning of major water sources enables a meaningful interpretation of an age dating tracer of the water fraction of interest and (ii) that the streambed has a major control on the estimated travel times.

Snowmelt contributes a significant fraction of groundwater recharge in snow‐dominated regions, making its accurate quantification crucial for sustainable water resources management. While several components of the hydrological cycle can be measured directly, catchment‐scale recharge can only be quantified indirectly. Stable water isotopes are often used as tracers to estimate snowmelt recharge, even though estimates based on stable water isotopes are biased due to the large variations of δ²H and δ¹⁸O in snow and the difficulty to measure snowmelt directly. To overcome this gap, a new tracer method based on on‐site measurements of dissolved He, ⁴⁰Ar, ⁸⁴Kr, N₂, O₂ and CO₂ is presented. The new method was developed alongside classical tracer methods (stable water isotopes, ²²²Rn, ³H/³He) in a highly instrumented boreal catchment. By revealing (noble gas) recharge temperatures and excess air, dissolved gases allow (i) the contribution of snowmelt to recharge, (ii) the temporal recharge dynamics, and (iii) the primary recharge pathways to be identified. In contrast to stable water isotopes, which produced highly inconsistent snowmelt recharge estimates for the experimental catchment, dissolved gases produced consistent estimates even when the temperature of snowmelt during recharge was not precisely known. As dissolved gases are not controlled by the same processes as stable water isotopes, they are not prone to the same biases and represent a highly complementary tracer method for the quantification of snowmelt recharge dynamics in snow‐dominated regions. Furthermore, an observed systematic depletion of N₂ in groundwater provides new evidence for the pathways of biological N‐fixation in boreal forest soils.

To provide a sound understanding of the sources, pathways, and residence times of groundwater water in alluvial river-aquifer systems, a combined multitracer and modeling experiment was carried out in an important alluvial drinking water wellfield in Switzerland. ²²²Rn, ³H/³He, atmospheric noble gases, and the novel ³⁷Ar-method were used to quantify residence times and mixing ratios of water from different sources. With a half-life of 35.1 days, ³⁷Ar allowed to successfully close a critical observational time gap between ²²²Rn and ³H/³He for residence times of weeks to months. Covering the entire range of residence times of groundwater in alluvial systems revealed that, to quantify the fractions of water from different sources in such systems, atmospheric noble gases and helium isotopes are tracers suited for end-member mixing analysis. A comparison between the tracer-based mixing ratios and mixing ratios simulated with a fully-integrated, physically-based flow model showed that models, which are only calibrated against hydraulic heads, cannot reliably reproduce mixing ratios or residence times of alluvial river-aquifer systems. However, the tracer-based mixing ratios allowed the identification of an appropriate flow model parametrization. Consequently, for alluvial systems, we recommend the combination of multitracer studies that cover all relevant residence times with fully-coupled, physically-based flow modeling to better characterize the complex interactions of river-aquifer systems.